1. Field of the Invention
The present invention relates to a spark plug for an internal combustion engine which can prevent lateral sparking.
2. Description of the Related Art
Conventionally, a spark plug for ignition is used in an internal combustion engine. In the spark plug, in general, a ground electrode is welded to a combustion-chamber-side tip portion of a metal shell which holds an insulator in which a center electrode is inserted. The other end portion of the ground electrode is opposed to the tip face of a tip portion of the center electrode, whereby a spark discharge gap is formed. When a spark discharge is caused between the center electrode and the ground electrode, an air-fuel mixture between the two electrodes is ignited and a flame nucleus is formed (refer to JP-A-2004-207219, for example).
If a rich air-fuel mixture is introduced continuously to the cylinder during operation of an internal combustion engine or if the internal combustion engine operates at low speeds over a long period of time, smoldering (smoldering pollution) where carbon adheres to an insulator surface around the tip portion of the center electrode may occur due to insufficient atomization of the fuel, temperature reduction of the insulator, or another reason. In the event of smoldering, current flows via the carbon adhered to the insulator surface, which may cause lateral sparking between the insulator surface and the inner circumferential surface of the metal shell. An effective measure against lateral sparking is to determine the clearance between the outer circumferential surface of the insulator and the inner circumferential surface of the metal shell and the length of the spark discharge gap so that a spark discharge occurs at the spark discharge gap even in the event of smoldering.
However, in recent years, the output power and fuel efficiency of automobile engines have increased and miniaturization of spark plugs has come to be required in order to secure a high degree of freedom in engine-side designing. Accordingly, the clearance between the outer circumferential surface of the insulator and the inner circumferential surface of the metal shell has been decreased, such that lateral sparking tends to occur at lower voltage differences than before. In particular, since the electric field strength is high around the ground electrode which projects from the tip face of the metal shell, spark plugs in which the dimensions of individual parts are merely scaled down from those of older versions are problematic in that a spark discharge tends to occur from the outer circumferential surface of the insulator to a ground-electrode-side portion of the inner circumferential surface of the metal shell in the event of smoldering.